1. Field of the Invention
This invention relates generally to a process and apparatus for forming a layer of mercury cadmium telluride, and, more particularly, to such a process using liquid phase epitaxial deposition from a mercury-rich melt.
2. Description of the Prior Art
Mercury cadmium telluride is a material which is useful as a photodetector, i.e., a detector that responds to radiant energy. In particular, mercury cadmium telluride (HgCdTe) is very useful in the fabrication of infrared detectors, i.e., devices which, in combination with other suitable signal processing means, are used to determine the presence and/or bearing of an object by sensing infrared radiation from its surfaces. More recently, infrared focal plane array technology has advanced toward single chip structures which incorporate multiple levels of photodetector cells, in which material properties are different on each level. Some multi-layered structures for infrared detection have been demonstrated in compound semiconductors of the elements in the periodic table in Groups IV and VI such as lead tin telluride (PbSnTe), and of Groups III and V such as indium arsenide antimonide (InAsSb), as described respectively, for example, by Schoolar et al in Infrared Physics, Vol. 20, No. 4, July 1980, page 271 et seq. and by D. T. Cheung et al in Applied Physics Letters, Vol. 30, No. 11, June 1977, page 587 et seq. However, compound semiconductors of Groups II and VI of the periodic table, such as HgCdTe, offer numerous potential advantages for multi-layered detection and signal processing. Such advantages include a wide range of applicability for wavelengths in the range from less than one micrometer to greater than twenty micrometers, a small lattice mismatch between mercury telluride (HgTe) and cadmium telluride (CdTe), low carrier concentrations, low dielectric constant, and low thermal expansion. In order to achieve these advantages, the HgCdTe must be formed as a uniform and highly pure layer and must also be formed in a controlled manner.
Liquid phase epitaxy (LPE) has been used to grow HgCdTe layers, using a process and apparatus as described, for example, by C. C. Wang, S. H. Shin, M. Chu, M. Lanir, and A. H. B. Vanderwyck, in the publication entitled "Liquid Phase Growth of HgCdTe Epitaxial Layers," In the Journal of the Electrochemical Society: SOLID-STATE SCIENCE AND TECHNOLOGY, January 1980, Vol. 127, No. 1, pages 175-178. By the process of Wang et al, a CdTe substrate is exposed to a tellurium solution containing predetermined amounts of mercury and cadmium, with the solution being maintained at a predetermined temperature above its liquidus temperature. Then, the solution is cooled below its saturation equilibrium temperature to cause the epitaxial crystal growth on the substrate. One major disadvantage of a process such as Wang's is that a high pressure system must be used in order to control the high mercury vapor pressure which occurs at the relatively high growth temperatures (e.g., 500.degree. C. or higher) required to maintain the crystal growth solution in a molten form.
A similar liquid phase epitaxial process for growing HgCdTe is disclosed in U.S. Pat. No. 3,902,924 to R. B. Maciolek et al, in which there is formed a nearly saturated liquid solution of mercury, cadmium, and tellurium which has a liquidus temperature that is substantially identical to the solids temperature of the desired (Hg,Cd)Te composition. The liquid solution is brought in contact with the substrate and the solution is cooled to produce supersaturation which results in the growth of a thin layer or film of (Hg,Cd)Te on the substrate. The process of Maciolek et al also has the disadvantage that high mercury pressures result from the relatively high growth temperatures (i.e., 800.degree. C. or higher) which are used. Since the mercury is constantly vaporizing from the melt, the composition of the crystal growth melt is constantly changing, and thus the epitaxial layer grown from this changing melt does not have a uniform composition.
Additional complications which arise from the high growth temperatures used in these prior art processes described above include a lack of clearly defined interface between multiple layers and between the substrate and the adjacent deposited layer due to diffusion effects, and an inability to adequately control the carrier concentration of the layers as grown.
It is the alleviation of these prior art problems concerning the formation of a uniform, high purity layer of HgCdTe and concerning the adverse affects caused by high crystal growth temperatures, to which the present invention is directed.